1. Field of the Invention
The invention relates to a method for the temporal filtering of the noise in an image of a sequence of digital images as well as to a device for carrying out this method.
The invention can be used notably for the processing of medical images formed in an X-ray fluoroscopy mode by means of a system which utilizes a low-intensity X-ray beam and produces a sequence of noisy digital images of low contrast which must be filtered so as to remove the noise without degrading the details.
The temporal filtering operation consists in smoothing, as a function of time, a monodimensional signal which is referred to as a temporal signal and consists of the digitized intensity values of a pixel having a given location in the images of the sequence.
2. Description of Related Art
A temporal filtering method of this kind is already known from the publication by R. E. KALMAN entitled "A new approach to linear filtering and prediction problems", published in "Transactions of the ASME", Journal of Basic Engineering, Series 82D, pp. 35-45, 1960.
Kalman filtering is defined by a recursive equation which yields the filtered intensity of a current pixel of an image of the sequence as a function of hypotheses made a priori, as a function of the intensity of the pixel having the same location in the preceding image of the sequence, and as a function of a factor which is called a Kalman gain.
This equation can lead to several recursive algorithms. In one of these algorithms the Kalman gain is a function of the difference between the noisy intensity observed at a given instant and the filtered intensity observed at the preceding instant for a pixel of given location. If an intensity discontinuity occurs due to a movement or a noise peak, therefore, the temporal signal is smoothed ahead of the discontinuity, but not after the discontinuity. Therefore, this algorithm is not suitable for dealing with this type of intensity discontinuity.
It is a problem that, due to the very low intensity of the fluoroscopic X-ray beam, the images of the sequence are extremely noisy and frequently contain noise peaks.
It is another problem that, due to the fact that each image of the sequence is separated from the next image by a small time interval, an important event, for example the displacement of a small tool such as a catheter, may occur from one image to another. The filtering of the image in which the movement of this small object occurs should not distort or erase this object.